The Stirling-cycle engine was first conceived and reduced to practice in Scotland 164 years ago. A hot-air, closed cycle prime mover based on the principle was patented by the Reverend Robert Stirling in 1817 as an alternative to the explosively dangerous steam engine. Incredibly, this event occurred early in the Age of Steam, long before the invention of the internal combustion engine and several years before the first formal exposition of the Laws of Thermodynamics.
Air was the first and only working fluid in early 19th century machines, whereas hydrogen and helium have been the preferred working fluids for modern machines. In Great Britain, Europe, and the United States thousands of regenerative hot air prime movers in a variety of shapes and sizes were widely used throughout the 19th centrury. The smaller engines were reliable, reasonably efficient for their time, and most important, safe compared with contemporary reciprocating steam engines. The larger engines were less reliable, however, because they tended to overheat and often succumbed unexpectedly to premature material failure.
Toward the end of the 19th century the electric motor and the internal combustion engine were developed and began to replace not only the Stirling-cycle engines, but also the reciprocating steam engines of that era. These new machines were preferred because they could produce greater power from more compact devices and because they were more economical to manufacture. The limitations of early, as well as those of current, Stirling engines are in part directly attributable to the design and performance characteristics of the structural materials used. Both the specific power capacity and the overall thermal efficiency of regenerative thermal machines are direct consequences of the inherent performance characteristics of their heat transfer elements.
Since World War II there have been unprecedented advances in the general technologies of machine design, heat transfer, materials science, system analysis and simulation, manufacturing methods, and Stirling engine development. Today, in comparison to their conventional internal combustion counterparts, all modern Stirling-cycle prime movers are external combustion engines which consistently demonstrate (in the laboratory) higher efficiency, multifuel capability, lower exhaust emissions, quieter operation, equivalent power density, and superior torque characteristics.
Nevertheless, none of these engines is mass produced for any commercial application anywhere in the modern world. The reason for this is that contemporary Stirling engines have been developed largely by adapting traditional methods and designs from the more familiar internal combustion engine technology base. Patchwork adaptation of the old as a shortcut to the new is a process which inexorably produces a hodge-podge arrangement of excessive mechanical complexity and which inevitably results in high production costs.
The use of ordinary engine construction materials such as cast iron or aluminum alloys, for example, often results in unnecessary thermal losses and unacceptable thermal stresses between heat transfer elements and adjacent components. Prior art solutions to these problems have resulted in increased complexity and cost. Despite clearly superior technical performance characteristics, therefore, contemporary Stirling engines are invariably not cost competitive from the standpoint of economical mass production.